It is not a discount the insurer gives you
The premium tax credit is a federal tax credit. What makes it feel like a discount is that you can take it in advance and have it paid straight to the insurer each month, so you only ever see the reduced premium. That mechanism is why the prices on a comparison site look absurd until you enter your income, and why two neighbours can pay very different amounts for the identical plan.
The benchmark plan sets the maths
Your credit is worked out from the second-lowest-cost Silver plan available in your county, known as the benchmark. Your expected contribution is calculated from your household income and size, and the credit is the difference between that contribution and the benchmark premium. You can then apply that credit to any metal tier you like, which is why a Bronze plan can end up costing very little and a Gold plan can end up surprisingly close to Silver.
County matters more than people expect
Because the benchmark is local, the same household income produces a different credit in Miami-Dade than in Monroe. Carrier participation is set county by county too. This is the practical reason a friend's advice about "the good plan" often does not transfer, even from one side of the county line to the other.
The second discount: cost-sharing reductions
If your household income falls roughly between 100% and 250% of the federal poverty level, Silver plans give you a quiet upgrade: a lower deductible, lower copays and a lower out-of-pocket maximum, all at the ordinary Silver premium. The insurer is reimbursed for the difference. This attaches to Silver and to nothing else, which is why the advice "buy the cheapest plan" is wrong for a large share of the households we see.
What reconciliation means
Because the credit is based on an estimate, it is trued up on your tax return. Earn less than you estimated and you may be owed more. Earn more and you may repay some of it. There are caps on that repayment at lower incomes, but they are not a licence to guess low. The right move is a realistic estimate and a phone call whenever your income changes materially.
Things that change your credit mid-year
A raise, a lost job, a new contract, a marriage, a birth, someone moving in or out of the household, or a move to a different county. All of them should be reported. Reporting an increase promptly means a smaller bill in April. Reporting a decrease promptly means more help now, when it is useful.
Where people leave money behind
Three places, consistently. Not entering income before comparing, so they never see their real prices. Picking Bronze while sitting in the cost-sharing range. And letting a plan auto-renew when the benchmark in their county has moved, which changes their credit without changing anything they did.